The recording densities of optical disks have been increasing from 0.6 GB of CD to 4.7 GB of DVD. However, along with progress in information technology, the development of an optical disk market is remarkable, and the emergence of a high density optical disk capable of storing a larger amount of data is desired. For example, an optical disk with a recording density of 100 Gbit/inch2 or higher which can accommodate to digital broadcasting is desired.
To increase the density of an optical disk, a distance between grooves or pits, i.e., a track pitch, is narrowed, thereby increasing a recording density in a track direction. For example, an increase in recording density from CD to DVD has been achieved by narrowing the track pitch from 1.6 μm to 0.74 μm.
An optical disk substrate is produced by injection-molding (injection-compression-molding) a thermoplastic resin. At that time, fine pits and projections formed in advance on a stamper attached to a mold and corresponding to recording/reproduction signals are transferred onto the surface of the substrate from the stamper. Thus, at the time of molding the substrate, transferring the pits and projections of the stamper with high precision, i.e., precise transferability, is important. Particularly, in molding a high density optical disk substrate, the precise transferability is very important.
For high density optical disks, it is important to comprise a substrate with high transferability. In addition to this, it is also important to undergo smaller warpage of a substrate and smaller warpage with respect to environmental changes than conventional optical disks, due to the following reason. That is, along with an increase in density, the wavelength of laser is made shorter and the NA of pickup lens is made higher, so that even very small warpage of a substrate results in a large coma aberration, thereby causing a focus error or a tracking error. Further, since the pickup lens and the substrate become closer to each other due to the increase in NA, it is important that the warpage of the substrate and the warpage caused by environmental changes are small, so as to prevent the lens and the substrate from making contact with each other.
Heretofore, a polycarbonate resin has been used as a material of substrates for optical disks such as CD (Compact Disk), MO (Magnetooptical Disk) and DVD (Digital Versatile Disk) due to its excellent transparency, heat resistance, mechanical properties and dimensional stability.
However, along with an increase in the recording densities of the optical disks, optical disk substrates made of the polycarbonate resin have been becoming unsatisfactory in view of precise transferability and warpage.
To meet demand for an improvement in transferability, a variety of studies have heretofore been made in terms of both molding techniques and material modification. As for the former, for example, it has been confirmed that a method of setting a cylinder temperature and a mold temperature at the time of substrate molding at high temperatures is effective. However, this method requires long cooling time in a mold because it is high temperature molding, thereby extending a molding cycle and resulting in low productivity. If molding is forcibly carried out in a high cycle, improper mold release occurs when a substrate is released from a mold, thereby resulting in deformed pits or grooves and lowering precision of transfer. As for the latter, for example, a method (JP-A 9-208684 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A 11-1551, for example) comprising incorporating a large amount of a low molecular weight compound in a polycarbonate resin so as to impart high flowability or a method (JP-A 11-269260, for example) comprising using a specific long chain alkyl phenol as a terminal blocking agent is proposed. However, in the method of increasing the content of the low molecular weight compound or the method of modifying terminal groups by the long chain alkyl phenol, deterioration in thermal stability is generally remarkable. Therefore, as a result of promoting thermal decomposition at the time of molding, the mechanical strength of a disk substrate is significantly degraded, so that the substrate is cracked by force which pushes the substrate out of a mold or the substrate is broken during handling of the optical disk substrate.
Further, to improve transfer of pits and projections to the surface of a polycarbonate resin and warpage, a resin composition containing a biphenyl compound or a terphenyl compound (particularly an orthoterphenyl compound or a metaterphenyl compound) in a given amount is proposed (JP-A 2000-239513). When this resin composition contains the biphenyl compound, the amount of deposits on a mold is large, resulting in low productivity, while when the composition contains the terphenyl compound, transferability is not improved to a fully satisfactory level.
Thus, the prior arts are intended to improve transferability by improving the flowability of the resin. However, they fail to provide practicable substrates at high efficiency.
Meanwhile, to meet demand for an improvement in warpage as well, a variety of studies have been made in terms of both molding techniques and material modification. As for the former, although warpage of a substrate can be kept small by finely controlling molding conditions, it is difficult to transfer the shape of a stamper precisely. As for the latter, it is known to be effective to use a rigid material having a high flexural or tensile modulus. Thus, for the purpose of improving the rigidity of a polycarbonate resin, a method of adding such additives as glass fibers and a filler has been attempted. However, although the above additives improve the rigidity of the polycarbonate resin, they are exposed to the surface, thereby degrading precision of transfer.
“Precise transferability” in the present invention refers to a characteristic that fine pits and projections formed on a stamper can be transferred precisely when an optical disk substrate is produced from a thermoplastic resin molding material by injection molding.
As for warpage with respect to environmental changes, JP-A 2000-11449 proposes “a disk-shaped data recording medium which comprises a substrate, a recording layer disposed on the substrate so as to record data signals and a transparent protective layer laminated on the recording layer and on which data signals are recorded/reproduced by light entering from the transparent protective layer side, the substrate comprising a resin core layer and a resin surface layer which is integrated with the core layer, has pits and projections corresponding to data signals of the recording layer side on one surface and has higher flowability than the core layer, the surface layer of the substrate being made of a resin having a water absorption of not higher than 0.3%”, so as to inhibit deformation caused by absorption of water, and suggests a complex substrate configuration by coinjection molding or sandwich molding so as to solve the problem.
Meanwhile, a light guide plate is an optical member used in planar light equipment attached to various displays such as a liquid crystal display. In an edge light mode, the light guide plate serves to allow light from a light source to eject in a direction perpendicular to the injection direction. Further, on the surface of the light guide plate, fine pits and projections which reflect or refract light efficiently are formed. Since such a light guide plate serves as a light source of a display, the member must have high permeability so as to achieve high brightness and uneven brightness on the light emitting surface must be low so as to achieve a uniform outgoing light quantity.
In recent years, as the size and thickness of displays have been increasing and decreasing year by year, respectively, the light guide plate has also been shifted to a larger size and a smaller thickness. When the light guide plate is produced by an injection molding method, the distance between the gate and the end of flow becomes longer along with an increase in size, so that pressure does not work effectively at the end of the flow. Further, along with a decrease in thickness, the progression of solidification of a molten resin by cooling is accelerated, so that pressure does not work effectively at the end of flow as in the case of the increase in size. Thus, there occurs a problem that transferability of pits and projections at the end of the flow is poor.
Meanwhile, displays such as a liquid crystal display are increasingly used in the outdoor. In this case, light guide plates are warped by a change in humidity of surrounding, thereby causing uneven display or interference with other components.